The present invention relates generally to rotary mediums for recording thereon information signals and to apparatuses for recording the information signals thereon. More particularly, the invention relates to an information signal recording medium of rotary type on which, without positively providing a groove for guiding a reproducing stylus, a main information signal is recorded on a track, and reference signals for the purpose of controlling the tracking of reproducing means for tracing and reproducing the recorded track of the main information signal are also so recorded as not to impart an adverse effect on the main information signal. Moreover, the invention relates to an apparatus for recording the main information signal and reference signals on this recording medium in such arrangement.
The systems known heretofore for recording and reproducing information signals such as video signals and/or audio signals on and from disc-shaped, rotary mediums (referred to hereinafter simply as discs) are broadly divided into systems in which reproduction is carried out optically, systems in which reproduction is carried out by means of a reproducing stylus employing a piezoelectric element, and systems in which reproduction is carried out by utilizing variations in the electrostatic capacitance between an electrode provided on the reproducing stylus and the recorded surface of the disc.
We have considered the advantages and disadvantages of these three kinds of systems and have adopted the electrostatic capacitance system as being the most desirable. Further, the present applicant has previously proposed, as a new system for overcoming the problems accompanying known electrostatic capacitance system, a novel "Information signal recording and reproducing system" as disclosed in the specification of U.S. patent application Ser. No. 785,095 filed on Apr. 6, 1977 assigned to the same assignee as in the present application.
More specifically, in accordance with a known electrostatic capacitance system, recording is accomplished in the recording system by forming a spiral guide groove in the disc for guiding the reproducing stylus and, at the same time, forming pits in responsive correspondence with an information signal to be recorded on the bottom surface of the groove. In the reproducing system, the reproducing stylus, being guided by the guide groove, traces the track within the groove and thereby reproduces the signal in response to variations in the electrostatic capacitance.
In this known system however, since a reproducing stylus guiding groove is provided in the disc, and the reproducing stylus is compulsorily guided by this guide groove, it is not possible for the reproducing stylus to undergo operations such as riding over the groove wall of one track and moving into another track and returning to the original track. If the reproducing stylus were to be forced to undergo such an operation, the groove and the stylus would be damaged. For this reason, it has not been possible in this known system to carry out special reproduction such as quick-motion picture reproduction, slow-motion picture reproduction, and still-picture reproduction.
Furthermore, since the area of contact between the tip of the reproducing stylus and the guide groove of the disc is small, the stylus easily wears. When the width of the stylus tip reaches the groove width as a result of abrasive wear, the serviceable life of this reproducing stylus ends. Thus, this system is accompanied by the problem of short life of the reproducing stylus. Another problem is that fine chip particles abraded from the disc by the tracing action of the reproducing stylus along the guide groove give rise to difficulties such as further promotion of the wear and damage of the stylus and the guide groove and an increase of the gap between the electrode at the stylus tip and the recorded surface including the pits within the disc guide groove, whereby the reproduced output decreases because of spacing loss.
If the rotational speed of the disc is set at a low value such as 450 rpm., for example, in order to prolong the life of the stylus tip, the recording wavelength of the information signal recorded in the guide groove will become short. For this reason, a recording apparatus of low price in which laser light is used cannot be employed in the recording system, and the disadvantageous employing of a recording apparatus of high price using an electron beam becomes unavoidable.
Accordingly, with the aim of overcoming the various problems of the known systems described above, the present applicant has proposed by the aforecited patent applications a novel system for recording and reproducing information signals. According to this proposed system, in the recording system thereof, pits are formed in accordance with the information signal being recorded along a spiral track on a recording medium of flat disc shape, without forming a groove therein, and, in the reproducing system, a reproducing stylus traces over and along this track thereby to reproduce the recorded information signal. In this proposed system, pilot or reference signals are recorded on or in the vicinity of a track of the information signal such as video signal on a rotary disc. At the time of reproducing, the reference signals are reproduced together with the video signal, and tracking servo control is carried out so that the reproducing stylus traces accurately along the track in response to the reproduced reference signals.
By the use of this previously proposed system, since the recording track has no groove, there is no possibility whatsoever of the reproducing stylus or the recording medium being damaged, and the stylus can trace the same portion of the track repeatedly a plurality of times, whereby a special reproduction such as still, slow motion, or quick motion reproduction becomes possible. Furthermore, other difficulties of the known system are removed.
Furthermore, as a reproducing stylus of highly desirable characteristics for reproducing an information signal recorded on a disc without a reproducing stylus guide groove, the present applicant has previously proposed a novel "Stylus for reproducing information signals recorded on a recording medium" as disclosed in the specification of U.S. patent application Ser. No. 852,366 filed on Nov. 16, 1977 and assigned to the same assignee as in the present application now U.S. Pat. No. 4,199,782. This proposed reproducing stylus has a bottom surface with a maximum width portion which is greater than the track pitch and has an electrode of a width smaller than the track pitch over a specific distance from the stylus tip.
A feature of this proposed reproducing stylus is that the stylus pressure per unit area is small, and the rate of abrasive wear of the reproducing stylus and the disc is low. Another feature of this stylus is that, since the electrode width does not increase appreciably even when the stylus tip becomes somewhat worn after a long period of use, the life of the reproducing stylus is remarkably long.
Thus, by using the recording and reproducing system and the reproducing stylus according to the above cited proposals, the problems encountered in the prior art are overcome, and very good results can be obtained.
On the other hand, the input-output characteristic of a reproducing apparatus for reproducing a rotary information signal recording medium, in general, is nonlinear, and the reproduced information signal and the reference signals are caused by this nonlinear characteristic to undergo cross modulation, giving rise to nonlinear distortion. For this reason, in the case where the information signal is a video signal, the picture quality of the reproduced picture deteriorates, while, in the case of an audio signal, the tone quality of the reproduced sound deteriorates.
One possible measure for preventing distortion (interference distortion) due to the above mentioned cross modulation is to effect an improvement of the linearity of the reproducing system. However, it is very difficult to cause the characteristics of the reproducing system to be perfectly linear, and a system so adapted would entail an extremely high cost.
The magnitude of the interference distortion occurring in the reproduced signal in the reproducing system corresponds to the difference between the levels of the information signal and the reference signals. Accordingly, by causing the level of the reproduced reference signal to be lower than the level of the reproduced information signals and, moreover, by causing the above level difference to be greater than a specific level difference, the magnitude of the interference distortion which develops can be reduced to a degree which is not detrimental for practical purposes.
However, in the case of reproduction of signals recorded by forming pits in the disc, the magnitude of the level of a reproduced output signal increases with the wavelength of signal recorded on the disc, increases with the pit depth up to a certain depth, and, furthermore, increases with the pit width (in the case where a reproducing stylus having an electrode width corresponding to the pit width is used). Therefore, by suitably selecting values such as the width and depth of the pits, the level of a reproduced output signal can be set at a desired value.